In the printing art, ink is applied to paper in the image or printed areas directly by contact with a printing plate or indirectly by intermediate transfer from a printing blanket which, in turn, is contacted by a printing plate. In any event, the inked image areas of the printed paper correspond to areas of the printing plate which carry ink while the blank unprinted areas of the paper correspond to areas of the plate which do not carry ink. The three main types of printing processes differ from one another on the basis of whether the ink is above, on or below the general surface from which it is applied. In the typographic, letterpress or relief process, the image areas or type are upraised above the general surface of the plate and the ink is applied to these upraised areas while no ink is present on the depressed general surface of the plate. In the planographic or lithographic process, the ink is on the plane of the surface of the plate, the inked or image areas of the plate being ink receptive (generally oleophilic) while the uninked or non-image areas are ink repellant (generally hydrophilic). In the intaglio process (including gravure) the ink is in depressed areas of the plate while no ink is present on the upraised general surface of the plate. See "Printing Ink Technology," Leonard Hill Limited, London, 1958, pp. 1 to 4.
The very nature of printing makes it impossible, in most cases, to print photographs as such since continuous tone cannot be produced on printing machines. Instead, the image is broken up by using a "cross-line screen." This screen is placed in front of a negative in a camera when the picture is photographed. A negative is obtained of equally or regularly spaced dark elements or dots separated by light areas or opaque elements or dots separated by transparent areas, or a positive is obtained of equally or regularly spaced light elements or dots separated by dark areas or transparent elements or dots separated by opaque areas. In any event, tonal effects or differences in tone are generally produced by varying the size of the elements or dots while maintaining an equal or regular spacing. The negative or positive can be printed down on the printing plate by exposing the plate to an appropriate light source through the negative or positive and etching or developing the plate to produce the elements or dots as raised or depressed areas or ink receptive or ink repellant areas. The ultimate paper print will then consist of an orderly array of equidistant, printed elements or dots which are small in the highlight areas but so large as to overlap in the dense areas. This "breaking-up" of a printed area is known as "half-tone" printing and produces a convincing visual or integrated optical density effect of tonal variations by virtue of changes in the element or dot sizes. The half-tone technique is used to print photographs in both the typographic and planographic processes. Newspaper half tones have about 80 dots to the linear inch, good quality illustrations have about 160 dots to the linear inch and half-tones as fine as 200 dots to the linear inch have occasionally been produced. While not referred to as half-tone printing, an analogous technique is used in printing photographs by the gravure (intaglio) process. In the gravure process, tonal effects are produced by varying the depth of etching of elements or dots, varying the size of elements or dots or both. Specifically, in rotogravure, etching of a copper base produces an assembly of square sunken cells of different depth for holding and delivering the ink. Paper prints from such plates excel in giving a good reproduction of the full tonal depth and detail of the original photograph. A magnifying glass reveals that the print is separated into squares of ink, and a super-imposed, unprinted cross-line screen covers the print, varying from 80 to 400 lines to the inch. See: Printing Ink Technology, supra, pp. 1 to 4 and pp. 302 - 303.
The above-mentioned arrangements of elements or dots used in printing photographs by half-tone or the like to produce a visual or integrated optical density effect of tonal variations are commonly referred to, by those skilled in the art, as "tints" and this terminology will be used in the present application.
The present invention is directed to a test strip for use in the preparation of printing surfaces or plates in which the surface or plate is coated with a photosensitive material which changes in character when light struck. The light struck areas may be photohardened or rendered soluble when struck by the light and either etched or developed, as appropriate, depending upon the coating and the type of plate being prepared. In preparing the plate for printing, again depending upon the nature of the coating, a negative or positive, consisting of patterns of dark and light or opaque and transparent areas corresponding to the configurations to be printed, is placed in contact with the photosensitive plate, the plate is then exposed to an appropriate source of light through the negative or positive and finally the plate is etched or developed. Specifically, in the photolithographic process, a light-hardened coating is disposed on the plate, the plate is exposed to light through a negative and the plate is processed or developed by dissolving away the unhardened areas with an appropriate developer solution. For purposes of simplification, further discussion herein will be confined to the photolithographic process. See Printing Ink Technology, supra, pp. 333 to 340.
In the lithographic art, when photosensitized printing plates are processed by hand, each plate is developed with a given amount of fresh developer. However, processing machines, for economic reasons, often use a tank of developer with a recirculation system. To maintain proper strength of the developer, replenisher usually must be added at certain intervals or at times the developer must be diluted. Primarily, replenishment recommendations are made based on the total square footage of plates processed. Such square footage recommendations are obviously quite inadequate due to aging of the developer, as a result of differences in the length of time that a developer is in the machine. Also, the average coverage of the image on the plate varies and, thus, the depletion rate of the developer varies. Other shop conditions further influence developer strength.
Variations in developer strength have the effect of either dissolving away too much or too little of the element or dot which carries the ink, thus resulting in a change in the size of the element or dot. Since half-tone printing of photographs depends upon variations in dot or element size to produce tonal effects or tint variations, such changes in element or dot size become quite critical and contribute to poor reproduction of the original.
Accordingly, in automatic processing, it is desirable to have an early warning device of some type, which will permit the operator to maintain constant developer activity in the processor regardless of individual conditions, in order to avoid wasting expensive plates.
In order to provide such an early warning device, all known existing test film targets were investigated to see if they would adequately serve the above purpose. In GATF Research Progress 69, "The GATF Dot Gain Scale" by Frank Preucil, Zenon Elyjiw and Robert F. Reed, published in November 1965, a dot gain scale designed for printing was described. This device was tested for purposes of determining its value as a test film target for monitoring developer activity and was found to have insufficient sensitivity to permit the operator to visually observe the result with the naked eye on the processed plate. Another device using a similar principle is the "Baker Signal Strip." The Baker Signal Strip is even less useful for purposes of monitoring developer activity than the GATF Dot Gain Scale. The Baker Signal Strip is used to detect dot gain but has only one sensitive area per target which can only give a yes or no answer. The Baker Signal Strip is described in Graphic Arts Monthly XXXVII No. 8, 1965, at page 126 by Eugene Bulinski. In the article, it is stated that "--a quick visual observation of the plate guide section does not give adequate judgement of the degree of gain."
It is therefore an object of the present invention to provide a test film target and a method of using the same. Specifically, the present invention provides a test film target to be used as a control guide for monitoring developer activity in automatic plate processors, to show dot size change in printing and dot size change in bi- and tri-metallic etching. Specifically, the control guide is in the form of a film which is laid-down or "stripped" on a border or other non-printing area of the negative or positive film, and exposed together with the original film to be reproduced on the photosensitized printing plate. Visual observation of the reproduced guide on the plate with the naked eye will permit readout of developer solution activity and provide a means for determining when replenisher must be added to the developer solution or a diluent for the developer fluid must be added long before any diminution or increase in the strength of the developer has any noticeable effect on the reproduction of the original film. As a matter of fact, the guide indicates developer solution activity to a degree which can be observed by the naked eye at normal viewing distances even though no visible change in the quality of the reproduced original can be observed by a 10 times magnifier.